The present invention relates to a cooling structure of an electric motor, and in particular relates to the cooling structure of the electric motor using a graphite sheet.
Conventionally, the electric motor has been used in various fields. The electric motor is rotated by converting supplied electric energy to kinematic energy. At this moment, part of the electric energy is converted to thermal energy, so that some heat is generated. Meanwhile, it is necessary to provide air insulation between an inner wall of a housing and a coil end of a stator from a safety perspective. Accordingly, the inner wall of the housing is configured to be spaced apart from the coil end of the stator, thereby providing a space portion between them. In this case in which there exists the space portion between the inner wall of the housing and the coil end, it is difficult to increase the thermal conductivity from the coil end to the housing, so that the temperature inside the electric motor tends to become high. In particular, the heat value of a large-sized (high capability) electric motor which is used for an electric car is high, which causes deterioration of the electric-motor efficiency. Therefore, various types of cooing technology have been proposed as its countermeasures.
A cooling structure of an electric motor disclosed in Japanese Patent Laid-Open Publication No. 2014-225971 comprises a rotor, a stator provided with a wound stator coil, a housing accommodating the rotor and the stator, a first reservoir for lubricant oil provided between a coil end positioned on an one-end side of the stator and the housing, a second reservoir for lubricant oil provided between another coil end positioned on the other-end side of the stator and the housing, and a connecting passage interconnecting the first and second reservoirs, wherein an upper-side portion of the connecting passage is located at a higher level than an oil surface of the lubricant oil stored in the first and second reservoirs. Thereby, the coil end is sunk into the lubricant oil, suppressing the lubricant oil from staying in the connecting passage, thereby performing cooling of the coil end.
In recent years, graphene (graphene sheet) has attracted attention as a material having a high mobility of charged particles (electrons). The graphene is a type of laminated crystal in which carbon atoms are coupled to each other in a hexagonal mesh shape, i.e., a type of graphite, and a single layer crystal having a thickness of a single atom is called the graphene, in particular. The mobility of the charged particles in the graphene is 15000 cm2/Vs at room temperature, and expression (exhibition) of characteristics relating to the mobility (electricity, heat, strength etc.) is limited to a plane direction.
The cooling structure of the electric motor disclosed in the above-described patent document can suppress the lubricant oil from staying in the connecting passage, without forming any ventilation hole inside the housing, by using the space portion for insulation as the reservoir of the lubricant oil. This cooling structure of the electric motor, however, is a so-called wet-type cooling in which the coil end is sunk into the lubricant oil (that is, a cooling type having heat exchange with the lubricant oil). Therefore, this type requires providing the lubricant oil and the oil passage for circulating the lubricant oil, and also needs maintenance of the lubricant oil and the like, so that there is a concern that manufacturing costs may increase improperly. Further, in a case of the large-sized electric motor, a temperature-increase speed is so fast (about 200° C. increases in 3 sec, for example) that there is a concern that the wet-type cooling by using the lubricant oil may lack a cooling capability for each component in the electric motor. Additionally, while the technology of the above-described patent document achieves further improvement of the cooling capability inside the electric motor by forming an additional cooling-water passage in a wall part of the housing in addition to the cooling mechanism by using the lubricant oil, there are still problems of a complex structure and costs and the like.
Since it is known that the graphene is superior in the thermal conductivity under a certain use condition as described above, it may be considered that the graphene or a graphene laminate (hereafter, these two will be referred to as “graphite sheet” collectively) is utilized for a cooling mechanism. In a case in which the graphite sheet is utilized for the cooling mechanism, it is necessary to set its application part, application form, or the like, considering specificity that physical property values are greatly different between a plane direction and a thickness direction (perpendicular-to-plane direction).
According to results of studies done by the present inventors, it has been found that even if the graphite sheet is face contacted to the coil end as a cooling object, the sufficient quantity of heat is not transmitted to the graphite sheet from the coil end unless the uniform face contact is done with a specified pressing force or greater, so that a desired cooling capability cannot be ensured. Herein, since a structure of the space portion for insulation where the coil end is arranged is narrow and complex, it is not easy to face contact the graphite sheet to the coil end with the uniform pressing force.